The present invention relates to a piston and cylinder for a vehicle engine wherein the piston has a finished exterior surface with a roughness total between approximately 6 and 8 micrometers and peak-to-peak distance of approximately 180 and 230 micrometers, and is coated with a composite coating to reduce scuffing.
Modern engines require tight clearance between pistons and cylinder bores for reduced noise, better fuel economy and reduced oil consumption. With the tight clearance, design for scuffing resistance between pistons and cylinder bores becomes a significant issue for automotive manufacturers because scuffing may cause engine failure.
Specifically, scuffing is an adhesive-wear event in which two parts slide against each other in a lubricant-starved condition. Piston skirt scuffing is characterized by a loss of the surface material and burnt or galled surfaces of the skirt and the cylinder bore. When an aluminum piston is used with a cast-iron cylinder bore, scuffing is characterized by the transfer of aluminum from the piston skirt to the cylinder bore surface.
Scuffing typically happens when the lubricating oil film at the interface is broken. The potential exists for this loss of lubrication due to overheating which causes the lubricating oil film to decompose, excessive force between the parts, or insufficient oil at the interface. Scuffing may happen whenever the engine is low on oil and/or low on coolant. Without sufficient coolant, the oil overheats and cannot sufficiently lubricate the piston/cylinder interface. Overfueling may also cause scuffing because the gasoline may wash away the lubricant from the piston surface. Oil pump failure or oil leakage may also result in scuffing because there is simply insufficient oil at the piston/cylinder bore interface.
Accordingly, it is desirable to provide an engine design with reduced scuffing between the piston and cylinder bore.
It has been surprisingly discovered that providing an exterior surface with turning marks between approximately 6 and 8 micrometers in depth and appropriately 180 and 230 micrometers in width with either a composite polymer coating or nickel-boron nitride composite plated coating thereon may substantially improve scuffing resistance between the piston and the cylinder bore.
More specifically, the invention provides a piston assembly for use in an engine. The assembly includes a piston body having a crown with a skirt extending from the crown. The skirt has an exterior surface. The exterior surface has a surface finish (or turning marks) in a wave form with peaks and valleys formed by a turning operation, and having a roughness total between approximately 6 and 8 micrometers. The roughness total is defined as the difference between the highest peak and lowest valley within an assessment length. The surface finish has an approximate peak to peak distance between 0.18 and 0.23 mm (180-230 micrometers) within the assessment length. A composite coating is provided over the finished exterior surface.
Preferably, the roughness total is approximately 7 micrometers, and the approximate peak to peak distance is 220 micrometers. The exterior surface is finished by a turning operation with a diamond-tipped cutting insert.
The composite coating may be a composite polymer coating (CPC) between approximately 10 and 16 micrometers in thickness. The composite polymer coating may be a polyamide resin having between approximately 5% and 30% by volume graphite particles, or a polyamide resin having between approximately 2% and 10% by volume graphite particles and between approximately 2% and 20% by volume molybdenum disulfide particles. The graphite and molybdenum disulfide particles are fibers with a length between approximately 3 and 15 micrometers and a diameter between approximately 1 and 5 micrometers.
Alternatively, the composite coating may be a Nixe2x80x94Pxe2x80x94BN plated coating including approximately 5% by volume BN (boron nitride) and approximately 3% by weight P (phosphorus). The Nixe2x80x94Pxe2x80x94BN coating has a thickness between approximately 12 and 17 micrometers and an approximate hardness of 50 HRC. The coating is electroplated and has suspended ceramic particles in the electroplating solution co-deposited during electroplating.
The cylinder bore may be prepared by a plateau-honing operation to provide bore surface with a roughness average, Ra, between approximately 0.34 and 0.52 micrometers.
A method is also provided for manufacturing a piston and cast-iron cylinder bore of an engine, wherein the piston includes a piston body having a crown with a skirt extending from the crown, and the cylinder is configured to receive the piston body. The method includes the steps of:
(A) finishing the exterior surface of the skirt in a turning operation with a transverse feed rate of between approximately 0.18 and 0.23 mm/revolution;
(B) applying a composite coating to the finished exterior surface; and
(C) honing a bore surface of the cylindrical bore to form a roughness average between approximately 0.34 and 0.52 micrometers.